1. Field of the Invention
The present invention relates to a vehicular transmission system including a master friction clutch and, in particular, to a vehicular transmission system and method for controlling operation of the transmission system as a function of system operating conditions.
2. Description of the Related Art
Vehicle transmission systems that include an internal combustion engine, a master friction clutch and a transmission are known in the art. A master friction clutch generally includes a cover assembly having a pressure plate, a cover and a diaphragm spring, all coupled for rotation together with an engine flywheel. A typical master friction clutch also includes at least one friction plate, commonly called a driven disc, having at its outer periphery friction pads and at its inner periphery a hub that is mounted on a driven member, such as a transmission input shaft. When the clutch is disengaged, the pressure plate is axially removed from the friction plate and no torque is transferred between the engine flywheel and the friction plate. When the clutch is engaged, the pressure plate applies a clamping force against the friction plate causing the engine flywheel and friction plate to contact and transfer torque therebetween.
Normally open (disengaged), centrifugally operated friction clutches are also well known in the art. Examples of centrifugally operated clutches may be seen by reference to U.S. Pat. Nos. 3,580,372; 3,580,372; 3,696,901; 5,437,356; 3,810,533; 4,819,779; 5,441,137; 5,730,269 and 4,610,343, the disclosures of which are incorporated herein by reference. Centrifugally operated clutches generally include an input member driven by an internal combustion engine and weights moveable with respect to the input member which, upon rotation of the input member, will move radially outwardly under the effect of centrifugal force to cause the input member to frictionally engage an output member. Centrifugal clutches dengage and  disengage as a function of engine speed, requiring manipulation of engine speed to selectively engage and disengage the clutch.
Unlike a normally closed (engaged) clutch, the pressure plate in a normally open (disengaged) clutch does not exert a clamping force on the friction plate(s) until the clutch is engaged. Thus, an installation device is generally required to temporarily engage the clutch during installation into a vehicular transmission system. In one such system, the installation device selectively engages at least one of the clutch weights to force the weight to move radially outwardly causing the input member to engage the output member. Once the clutch is installed in the vehicular transmission system, the clutch is disengaged by operating the engine at a predetermined speed that allows the engaged weight to move further radially outward and disengage from the installation device. With the weight successfully disengaged from the installation device, the clutch and vehicular transmission system are ready for normal operation. However, failure to operate the engine at the predetermined engine speed after clutch installation may undesirably result in the clutch remaining engaged.
Vehicular transmission systems may also include an inertia brake, which functions to synchronize the rotation speed of the transmission input shaft with the speed of the transmission output shaft. When launching a vehicle, the inertia brake slows or stops rotation of the input shaft to allow the transmission to engage a low gear ratio (e.g., the staring gear ratio). Failure of the clutch to disengage, such as when the engine has not been operated at the predetermined engine speed after clutch installation, may damage an activated inertia brake and other components of the vehicular transmission system. Moreover, failure of the clutch to disengage may cause the engine to stall when the inertia brake is activated.
For these and other reasons, it is desirable to provide an improved approach for operating a vehicular transmission system, particularly those that employ a centrifugal master friction clutch, to reduce or minimize the known limitations of the prior art.